The present invention relates to a method and an apparatus for recirculating exhaust gas and more specifically to a method and an apparatus for an engine needing a relatively high recirculation rate of exhaust gas.
As is well known, there are so many engines being equipped with an exhaust gas recirculation (hereafter referred to as "EGR") system in recent years. The EGR system is one of exhaust emission control systems by which NOx emission is reduced through recirculating a part of exhaust gas into an induction system and reducing the combustion temperature.
Commonly in the conventional EGR system, a take-out port of exhaust gas is disposed in the vicinity of an exhaust port of an engine and the exhaust gas is returned to an induction system of an engine through a control valve.
On the other hand, as disclosed in Japanese patent application laid open No. 1988-78256, it is proposed that in an EGR system in which a plurality of exhaust manifolds are provided and a catalytic converter is disposed corresponding to each exhaust manifold, a take-out port of exhaust gas is located at the downstream portion of the catalytic converter.
In a conventional engine where an air-fuel ratio control is carried out near the theoretical air-fuel ratio, an EGR rate (a rate of exhaust gas against induction air) has an upper limit near at most 20% because of a problem of combustion stability, although generally it is not necessary to recirculate so much amount of exhaust gas.
However, in a two cycle engine, especially a two cycle engine having a high pressure direct injection system (injecting fuel into a cylinder with high pressure) or a four cycle lean burn engine, a present 3-way catalyst which reduces CO, HC and NOx simultaneously at the theoretical air-fuel ratio is not effective. Therefore, an EGR is inevitably necessary to reduce NOx emission. In this case a very high EGR rate, for example, 40% to 50% of the EGR rate is needed. Namely, a large amount of recirculation gas is needed to reduce NOx emission.
Also, the lower the temperature of the recirculated exhaust gas is, the more the combustion temperature goes down, thereby the formation of NOx being suppressed.
Furthermore, since a two cycle engine with a high pressure direct injection system or a four cycle lean burn engine have a relatively high oxygen concentration in the exhaust gas, it is necessary to let flow exhaust gas more than several times as much as in case of a conventional engine, consequently the conventional EGR system comes substantially short of a flow capability in the recirculated exhaust gas.